Double-walled tank



FIG. 1 is a top and front and left side perspective view of the double-walled tank showing our new design;

FIG. 2 is top plan view thereof;

FIG. 3 is a front view thereof the rear view being a mirror image thereof; and,

FIG. 4 is a left side elevational view thereof the right side being a mirror image thereof.

The broken lines shown in the figures are for illustrative purposes only, and the subject matter depicted thereby constitutes no part of the claimed design. 

The ornamental design for a double-walled tank, as shown and described. 